history: make checkboxes always visible (instead of just when hovered).

history: fix a11y issue by making checkboxes always visible.

this change:
- changes border-color instead of opacity of checkboxes on hover
- makes checked and hovered checkboxes look more similar
- makes hovered rows more noticeable

And I touched a few more things that weren't really noticeable
before I changed the background color:
- checkbox vertical alignment
- added padding in front of checkboxes

BUG=351565
R=dmazzoni@chromium.org
TEST=visible checkboxes

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=272735

[dmazzoni, 2014-05-23 23:42:59]

lgtm

[Dan Beam, 2014-05-24 00:04:29]

The CQ bit was checked by dbeam@chromium.org

[commit-bot: I haz the power, 2014-05-24 00:05:10]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/dbeam@chromium.org/293303002/1

[commit-bot: I haz the power, 2014-05-24 13:13:12]

Change committed as 272735